DESCRIPTION (Adapted from applicant's description): Congenital malformations are a major cause of morbidity and mortality in infants and children. Knowledge of the genetic and molecular programs that govern normal development will improve our ability to understand, diagnose and eventually, prevent such malformations. Developmental programs that guide limb formation operate in other regions of the embryo; discoveries in mechanisms of limb development provide useful insight into how other organs and structures are formed. Fibroblast Growth Factors (FGFs) are signaling molecules that function in many critical developmental pathways. The importance of FGFs in human development was confirmed by discoveries that human syndromes, such as Apert, Pfeiffer, and Crouzon syndromes are caused by mutations in FGF receptors. The goal of the proposed project is to determine the roles of FGF4 and FGF8 during early limb development. Disruption of either Fgf8 or Fgf4 in the germline of mice results in early embryonic lethality, preventing examination of their roles in later developmental processes. To circumvent this lethality, mice were generated bearing conditional mutations of Fgf4 and Fgf8 that were inactivated in a developmentally-regulated and tissue-restricted manner. These mice survive, display limb phenotypes not predicted by existing models of FGF function and provoke new questions that are the basis of this proposal. Alterations in intercellular signaling, cell death and proliferation that result in limb defects in Fgf8 conditional mutants will be characterized. Possible functional redundancy of FGF8 and FGF4 will be tested by simultaneous disruption of these genes in the limb. To examine the role of Fgf8 in limb initiation, it will be ablated in the intermediate mesoderm of mouse embryos. Reagents developed to achieve these aims will also make the conditional mutagenesis system a powerful tool to investigate the role of these genes in other regions of the developing mouse embryo, including the cardiovascular system. Additional research training with Dr. Capecchi and the support of a MCSDA will prepare Dr. Moon to begin her independent research career investigating fundamental regulatory pathways in vertebrate development. Ultimately, this knowledge will help us to understand how alterations in those pathways, whether genetic or environmental, result in congenital malformations in infants and children.